Method for use in conjunction with an exhaust-gas aftertreatment system

ABSTRACT

A method for an exhaust-gas aftertreatment system which is operated on an internal combustion engine operated with an excess of air, in which exhaust-gas aftertreatment system soot particles are separated out of exhaust gas by a particle filter and the particle filter is regenerated with the aid of NO 2  formed on an NO oxidation catalytic converter mounted upstream of the particle filter. A reduction of the nitrogen oxides is realized by a reducing agent which splits to form ammonia being metered into the exhaust-gas flow upstream of the particle filter and the particle filter is laden with a catalytically active material for the selective catalytic reduction of nitrogen oxides. The NH 3  to NO x  ratio (feed ratio α) is varied in phases by changing the nitrogen oxide untreated emissions and/or the supplied amount of reducing agent, in such a way that the feed ratio α alternates in phases between values of greater than one and values of less than one.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority of DE 10 2012 006 448.9 filed Mar.30, 2012 which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for operating an exhaust-gasaftertreatment system in internal combustion engines operated with anexcess of aft, such as diesel engines and gasoline engines with directinjection.

2. Description of Prior Art

Aside from solid particles, nitrogen oxides are among the limitedexhaust-gas components which are generated during combustion processesand the permitted emissions of which are being progressively lowered.Various methods are currently used to minimize the exhaust-gascomponents in internal combustion engines operated in motor vehicles.The reduction of the nitrogen oxides is realized usually with the aid ofcatalytic converters; in oxygen-rich exhaust gas, a reducing agent isadditionally necessary in order to increase the selectivity and theNO_(x) conversion. These methods have become known under the collectiveterm SCR methods, wherein SCR stands for “selective catalyticreduction”. These methods have been used for many years in the field ofpower plants and recently &so for internal combustion engines. Adetailed explanation of such methods emerges from DE 34 28 232 A1. Inpractical applications, ammonia or compounds which split to formammonia, such as urea or ammonium formate, in solid form or in solution,are used as reducing agent. For the conversion from one mole of nitrogenmonoxide, one mole of ammonia is required, wherein the nitrogen oxidesreact with the NH₃ deposited on the catalytic converter in accordancewith the following equation:4NO+4NH₃+O₂

4N₂+6H₂O  (1).

The ratio between NH₃ and NO_(x) is referred to as the feed ratio α.α=NH₃/NO_(x)

In the case of an ideal catalytic converter, this means that, for a feedratio of α=1, all of the nitrogen oxides are reduced, that is to say100% NO_(x) conversion is attained, because the following applies forthe NO_(x) conversion X_(NOx):

$X_{NOx} = \frac{C_{{NOx},0} - C_{NOx}}{C_{{NOx},0}}$where: c_(NOx,0): NO_(x) untreated emissions [ppm]

-   -   c_(NOx): NO_(x) emissions downstream of catalytic converter        [ppm]

If a platinum-containing NO oxidation catalytic converter for formingNO₂ is positioned upstream of the SCR catalytic converters2NO+O₂

2NO₂  (2)then the SCR reaction can be accelerated considerably, andlow-temperature activity can be markedly increased.NO+2NH₃+NO₂

2N₂+3H₂O  (3)

In the case of internal combustion engines operated in vehicles, thenitrogen oxide reduction with the aid of the SCR method is difficultbecause fluctuating operating conditions prevail there, which makes thequantitative metering of reducing agent difficult. It is duly sought, onthe one hand, to attain as high as possible a conversion of nitrogenoxides; it must on the other hand be ensured that no emissions ofunconsumed ammonia occur. To remedy this, use is often made of anammonia blocking catalytic converter which is positioned downstream ofthe SCR catalytic converter and which converts excess ammonia intonitrogen and water vapour.

To minimize fine particle emissions, particle filters are used both inthe power plant field and also in vehicles. In particle filters, thediameter of the filter ducts lies in the range of the diameters of theparticles. Owing to this fact, particle filters are at risk of becomingblocked, which increases the exhaust-gas back pressure and reducesengine power. An arrangement and a method with a particle filter emergefrom EP 0 341 832 A2. The abovementioned arrangements and theabovementioned method are characterized in that the oxidation catalyticconverter—usually a catalytic converter with platinum as activematerial—arranged upstream of the particle filter oxidizes the nitrogenmonoxide in the exhaust gas, with the aid of the residual oxygenlikewise present in the exhaust gas, to form nitrogen dioxide, which inturn is converted in the particle filter with the carbon particles toform CO, CO₂, N₂ and NO. A continuous removal of the accumulated fineparticles is realized in this way; regeneration cycles such as must beperformed in a cumbersome manner in the case of other arrangements arethereby dispensed with.2NO₂+C

2NO+CO₂  (4)2NO₂+C

2NO+CO  (5)2C+2NO₂

N₂+2CO₂  (6)

If complete oxidation of the carbon stored in the particle filter is notattained with the aid of NO₂, the carbon fraction and thus theexhaust-gas back pressure progressively increase. If a critical mass isreached, an uncontrolled ignition of the carbon may occur at highexhaust-gas temperatures, which carbon then burns abruptly with oxygen.C+O₂

CO₂  (7)

This leads to a temperature rise to up to 1000° C.

The simultaneous use both of arrangements for reducing nitrogen oxideemissions and also arrangements for reducing fine particle emissions isnecessary in order to comply with the future exhaust-gas regulations.Various arrangements and methods for this purpose have already becomeknown.

DE 103 48 799 A1 describes an arrangement composed of an oxidationcatalytic converter, an SCR catalytic converter arranged downstream ofthe oxidation catalytic converter in the exhaust-gas flow, and aparticle filter arranged, in turn, downstream of the SCR catalyticconverter in the exhaust-gas flow. The supply of the reducing agent forthe selective catalytic reaction that takes place in the SCR catalyticconverter takes place directly upstream of the SCR catalytic converterby means of a urea injection device that is controlled as a function ofoperating parameters of the internal combustion engine. A disadvantageof this arrangement is that the nitrogen dioxide generated in theoxidation catalytic converter is substantially completely consumed bythe selective catalytic reduction in the SCR catalytic converter, thatis to say is not available for the conversion of the fine particles thathave accumulated in the downstream particle filter. The regeneration ofthe particle filter must therefore be effected in a cumbersome manner bycyclic heating of the exhaust-gas flow by virtue of the exhaust-gas flowbeing enriched with unburned hydrocarbons and these subsequently beingcatalytically oxidized. This is realized either through enrichment ofthe combustion mixture or the injection of fuel upstream of the particlefilter. Such an arrangement for the regeneration of the particle filteris firstly cumbersome and therefore expensive, and, secondly, the cyclicregeneration of the particle filter situated at the end of thearrangement generates new pollutants, such as carbon monoxide, which canbe removed from the exhaust gas again only with difficulty.

A further combination of a particle filter and an arrangement forselective catalytic reduction has become known from EP 1 054 722 A1. Thearrangement described therein is composed of an oxidation catalyticconverter, which is arranged in the exhaust-gas flow and which increasesthe fraction of nitrogen dioxide in the exhaust gas, a fine particlefilter arranged downstream, a reservoir for the reducing fluid, aninjection device for the reducing fluid, the injection device beingarranged downstream of the fine particle filter, and an SCR catalyticconverter arranged downstream of the injection device in the exhaust-gasflow. The arrangement described above duly permits a continuousconversion of the soot-type fine particles that have accumulated in thefine particle filter with the aid of the nitrogen dioxide generated inthe oxidation catalytic converter, but has another major disadvantage.The particle filter causes cooling of the exhaust gas, such that if forexample the commercially available reducing fluid named AdBlue is used,it is the case in particular after starting of the internal combustionengine or during operation of the internal combustion engine in thelower power range that the exhaust-gas temperature is too low togenerate ammonia from the 33% aqueous urea solution without theformation of problematic by-products.

In conjunction with the breakdown of urea ((NH₂)₂CO) to form ammonia(NH₃), it is known that, under optimum conditions (temperatures above350° C.), this takes place in two stages; firstly, the thermolysis, thatis to say the thermal breakdown of urea, takes place as per the equation(NH₂)₂CO

NH₃+HNCO  (8).

Subsequently, the hydrolysis, that is to say the catalytic breakdown ofisocyanic add (HNCO) to form ammonia (NH₃) and carbon dioxide (CO₂),takes place as per the equationHNCO+H₂O

NH₃+CO₂  (9).

Since, if AdBlue is used, the reducing agent is present in a formdissolved in water, the water must evaporate before and during theactual thermolysis and hydrolysis.

If the temperatures prevailing during the above reactions (7) and (8)are below 350° C. or if heating takes place only slowly, it is knownfrom DE 40 38 054 A1 that primarily solid, non-melting cyanuric add isgenerated by trimerization of the isocyanic add formed in equation (7),as per3H CO^(<350° C. . . .) _(. . . >350° C.)(HNCO)₃  (10),which cyanuric add leads to blockage of the downstream SCR catalyticconverter. This may be remedied, as stated in the cited DE 40 38 054, byvirtue of the exhaust-gas flow which is laden with reducing agent beingconducted across a hydrolysis catalytic converter. The exhaust-gastemperature above which quantitative hydrolysis becomes possible canthus be pushed to 160° C. The construction and composition of acorresponding catalytic converter is described in the cited publication,as is the construction and function of an SCR catalytic converter systemequipped with a hydrolysis catalytic converter. Such an additionalhydrolysis catalytic converter however additionally increases the costof the arrangement for exhaust-gas aftertreatment.

An exhaust-gas aftertreatment arrangement composed of an oxidationcatalytic converter and a downstream particle filter which is combinedwith an SCR catalytic converter to form a structural unit such that theparticle filter is coated with an SCR-active compound or is composed ofsuch a compound or has embedded SCR-active centres, is known from DE 10323 607. For the introduction of the reducing agent, it is provided inthis arrangement that a supply means for the reducing agent, preferablyan aqueous urea solution, is provided downstream of the oxidationcatalytic converter and upstream of the particle filter. Such anarrangement duly has the advantage of a small structural size and a lowheat capacity, but, as tests have shown, has the disadvantage that, as aresult of the ammonia storage property of SCR-active catalyst materials,a significant excess of ammonia can occur in those regions of thearrangement which are directly adjacent to the accumulated particles,such that the reaction as per equation (3) competes directly in a verylimited space with the reaction as per equations (4) to (6). Therefore,fewer of the accumulated carbon particles are converted than would benecessary in order to avoid an increase of the exhaust-gas back pressureand the risk of an uncontrolled burn-off of the accumulated carbonparticles as per equation (7) in the event of a critical particle massbeing reached. The first results in a power decrease if the exhaust-gasaftertreatment arrangement is installed in the exhaust tract of aninternal combustion engine, and the second results in possibledestruction of the entire arrangement.

A further arrangement for exhaust-gas aftertreatment emerges from EP2014348. This document specifies a method for the reduction of nitrogenoxides, which method comprises the following steps;

a) means for providing NO₂ in the exhaust gas which contains NO_(x) andsoot, wherein the means are engine-internal measures, and/or at leastsome of the exhaust gas which contains NO_(x) and soot is brought intocontact with an oxidation catalytic converter which increases theNO₂/NO, ratio;

b) an injection module which is designed to inject a predeterminedamount of ammonia at least into some of the exhaust gas which containsNO_(x) and soot, the ammonia being in the form of either pure ammonia ora precursor compound for ammonia such as urea, a liquid solution ofurea, ammonium carbamate, isocyanic acid, cyanuric acid, methaneamide,etc. or combinations of these, wherein the injection module is arrangeddownstream of or parallel to the oxidation catalytic converter in theflow direction of the exhaust gas which contains NO_(x) and soot, and

c) a device for the filtering/separation of soot, that is to say carbonparticles, wherein the NO_(x) contained in the exhaust gas and the sootare intended to make contact with the soot that has accumulated in thedevice for the filtering/separation of soot, that is to say the carbonparticles, with the ammonia-enriched exhaust gas which contains NO_(x)and soot, whereby a selective catalytic reduction of at least some ofthe NO_(x) molecules with the ammonia to form nitrogen and water isinitiated.

SUMMARY OF THE INVENTION

Taking the prior art described above as a starting point, it is anobject of the invention, while avoiding the disadvantages of knownarrangements, to specify an exhaust-gas aftertreatment system fornitrogen oxide and particle reduction in internal combustion enginesoperated with an excess of air, which exhaust-gas aftertreatment systemreduces both the particle emissions and the nitrogen oxide emissions,and the structural size of which exhaust-gas aftertreatment system isminimized through the combination of components and functions. It is afurther object in particular to ensure the regeneration of the particlefilter, and reliably prevent deposits from the reducing agent on theparticle filter, through suitable process implementation.

The object is achieved by means of an exhaust-gas aftertreatment systemand a method for the operation thereof as per the characterizing part ofclaim 1; advantageous refinements of the exhaust-gas aftertreatmentsystem are specified in the dependent claims.

For the achievement of the object, it was assumed that, to attain thedemanded exhaust-gas limit values, the nitrogen oxide reduction isrealized by means of an SCR reaction with the aid of ammonia and theparticle reduction is realized by means of a particle filter. Theexhaust-gas aftertreatment system according to the invention provides aparticle filter, which is a surface-type particle filter or a depthfilter, in the exhaust-gas flow of the internal combustion engine.Upstream of the particle filter there is mounted a catalytic converterfor the oxidation of NO to form NO₂ in order to ensure a regeneration ofthe particle filter by means of NO₂. For the implementation of theselective catalytic reduction of the nitrogen oxides, the particlefilter is laden with soot and/or an SCR-active catalyst material. Thesupply of a reducing agent which splits to form ammonia such as isrequired for the generation of ammonia, or the supply of ammonia, takesplace upstream of the particle filter. The abovementioned disadvantagesof such an arrangement are eliminated by means of the processimplementation described below: The supplied amount of reducing agentand/or the NO_(x) untreated emissions are/is periodically raised orlowered such that, in periods with a low feed ratio, the amount ofNO_(x) exceeds the supplied amount of reducing agent, which in turn hasthe result that a reaction of the NO_(x) with the ammonia that hasalready accumulated on the soot filter and/or on the SCR-active catalystmaterial takes place while NH₃ is no longer being provided in sufficientamounts to keep the ammonia loading of the soot filter and/or SCR-activecatalyst material constant, and therefore the ammonia loading thereofdecreases. In particular, the ammonia loading at the filter inlet and onthat side of the filter wall which faces towards the untreated exhaustgas decreases. Since the SCR reaction as per equations 1 and 3 howevertakes place via intermediate stages of NH₃ accumulated on the sootfilter and/or SCR-active catalyst material, this has the result that thefocus of the NO_(x) conversion is shifted from the filter inlet and theuntreated-gas side of the filter wall towards the filter outlet and theclean-gas side of the filter wall. In this way, the amount of NO₂ on theuntreated-exhaust-gas side of the particle filter is increased owing tothe reduced conversion taking place there owing to the local deficiencyof NH₃. As a result of the increased NO₂ availability, the oxidation ofthe soot that has been deposited on the untreated-exhaust-gas side ofthe particle filter is improved considerably. To nevertheless ensure ahigh NO_(x) conversion, it is provided that, after phases with low feedratio and before all of the ammonia stored in the soot filter and/orSCR-active catalyst material has been consumed, the supplied amount ofreducing agent is increased and/or the NO_(x) untreated emissions aredecreased such that the supplied amount of reducing agent exceeds theamount of NO_(x), that is to say a high feed ratio is present. As aresult of this measure, the soot filter and/or the SCR-active catalystmaterial is laden with NH₃ again, whereby a decrease of NO_(x)conversion rates can be prevented.

The advantage of the method according to the invention thus consists inlarge amounts of NO₂ periodically being present on thesoot-particle-containing untreated-exhaust-gas side, which large amountsof NO₂ permit a reliable oxidation of the carbon-containing soot, whilelarge amounts of NH₃ are subsequently stored in the soot filter and/orSCR-active catalyst material, whereby high NO_(x) conversion rates canbe attained.

To accelerate the “discharging” of ammonia from the catalytic converter,it is possible for the supply of reducing agent to be completelyprevented or at least reduced in the phases.

The NO_(x) untreated emissions may, as already described, be varied bychanging operating parameters of the internal combustion engine.Operating parameters which have a direct influence on the NO_(x)emissions include inter alia the start of injection of the fuel, the airto fuel ratio (lambda), the fuel injection pressure, the number andchronological sequence of fuel injections per working stroke, the intakeair temperature and the recirculated amount of exhaust gas (EGR rate) ifexhaust-gas recirculation is provided. The advantage of the reduction ofthe feed ratio by means of the increase of the NO_(x) untreatedemissions consists in that the NO₂ availability on the untreated-gasside is increased not only by the depletion of NH₃, that is to say thedegraded SCR reaction, but rather the amount of NO₂ supplied to theparticle filter is raised already owing to the increased NO_(x)untreated emissions, whereby the soot oxidation is additionallyaccelerated.

Since the ammonia storage and release behaviour is highly dependent onthe operating conditions of the exhaust-gas aftertreatment system, suchas temperature, degree of ammonia loading, NO_(x) conversion, NO_(x)untreated emissions, NO₂ emissions upstream of the particle filter,NO_(x) emissions downstream of the system, NH₃ emissions downstream ofthe system and supplied amount of reducing agent, it is advantageous forthe period length and/or the magnitude of the increase and/or themagnitude of the reduction and/or the duration of the increase and/orthe duration of the reduction of the NO_(x) untreated emissions and/orof the supplied amount of reducing agent to be made dependent on thevariables.

Aside from the ammonia storage and release behaviour, reliable particlefilter regeneration is essential for fault-free functioning of thesystem. For this reason, it is expedient for the period length and/orthe magnitude of the increase and/or the magnitude of the reductionand/or the duration of the increase and/or the duration of the reductionof the NO_(x) untreated emissions to also be made dependent on the sootloading of the particle filter.

The above-described variables may be determined either directly by meansof sensors, such as pressure, temperature, NO_(x), NH₃ and NO₂ sensors,or by means of models in the form of mathematical functions,characteristic maps and/or neural networks.

To keep the structural volume of the filter small, the catalyst isapplied predominantly in the gas-permeable regions of the filter, suchthat particle separation and NO_(x) reduction can take place in a verylimited space.

Materials which are advantageously suitable for the particle filter withregard to the loading with SCR-active catalyst material are cordieriteor silicon carbide or sintered metal or ceramic fibres or silicatefibres or metal meshes.

It is expedient, and thus advantageous, for the loading with SCR activecatalyst material on the outflow side of the particle filter to increasein the flow direction of the exhaust gas. The corresponding distributionof the SCR-active material can be attained by virtue of the particlefilter being built up in layers, wherein the outflow-side layers have aloading with the SCR-active material which increases in the flowdirection of the exhaust gas. It is self-evidently also possible for theloading with the SCR-active material to be realized by coating orimpregnating the outflow side of the particle filter such that thedegree of loading of the filter material decreases with progressivepenetration depth of the coating or impregnation solution. TheSCR-active catalyst material with which the particle filter is laden maycontain, as active component, vanadium(IV) oxide and/or V₂O₅ and/orWO₃/TiO₂ and/or iron-containing zeolites and/or copper-containingzeolites and/or cobalt-containing zeolites, wherein zeolite-containingcatalyst types are preferable owing to their high NH₃ storagecapability.

To promote the splitting of ammonia from the reducing agent, it isfurthermore advantageous for the particle filter to additionally becoated or impregnated on the inflow side with a correspondingly activecatalyst material for the formation of NH₃. As an active component for acatalyst material of this type, use may be made of TiO₂ and/or TiO₂/SiO₃and/or TiO₂/SiO₂/Al₂O₃ and/or zirconium oxides and/or zeolites.

To prevent any unconsumed NH₃ from passing into the environment duringthe SCR reaction, it is advantageous for the particle filter toadditionally be coated or impregnated on the outflow side, at the end ofthe loading with SCR-active catalyst material as viewed in the flowdirection of the exhaust gas, with a catalyst material which oxidizesexcess ammonia present in the exhaust gas. The catalyst materialsuitable for this purpose for oxidizing excess ammonia may containelements of the platinum group and/or the oxides thereof and/or zeolitesas active component.

One particularly expedient and thus advantageous embodiment of theparticle filter provides that the latter forms blind-hole-like orpocket-like chambers both on its inflow side and also on its outflowside, wherein those situated on the inflow side are open towards theinflow side and those on the outflow side are open towards the outflowside.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail below with the aid of thedrawings and on the basis of some examples. In the drawings:

FIG. 1 is a schematic illustration of an exhaust-gas aftertreatmentsystem.

FIG. 2 shows a combination of a particle filter and an SCR catalyticconverter in a schematic illustration.

FIG. 3 shows a combination of a particle filter and an SCR catalyticconverter in a schematic illustration with a detail cutout.

FIG. 4 is a table showing a comparison of the effects with and withoutan NO₂ increase.

FIG. 5 shows a graphic illustration of the comparison in FIG. 4.

FIG. 6 is a flow diagram of a method for an exhaust-gas aftertreatmentsystem that operates on exhaust gas of an internal combustion engineaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An exhaust-gas aftertreatment system for selective catalytic reductionand for the reduction of solid particles in the exhaust gas of aninternal combustion engine is shown in a schematic illustration inFIG. 1. The exhaust gases generated by an internal combustion engine(not illustrated) as a result of the combustion processes, the exhaustgases being indicated in FIG. 1 by the arrows, pass into the exhaust-gasaftertreatment system 1, in which the hot exhaust gas firstly flowsthrough an oxidation catalytic converter 4, the task of which is tooxidize some of the nitrogen monoxide contained in the exhaust gas toform nitrogen dioxide by means of the excess oxygen present in theexhaust gas, as per the reaction labelled (2) above. Downstream of theoxidation catalytic converter, a reducing agent is metered in at asdose-coupled a position as possible. The reducing agent is an aqueousurea solution, as is conventional in practice in motor vehicles with SCRreaction; it is however self-evidently also possible for urea in solidform to be metered in, as has already been described in detail in therelevant technical literature. Furthermore, it is possible for ammoniato be metered in as reducing agent, which ammonia is obtained at someother location, for example under more expedient thermal conditions,from a substance which splits to form ammonia. The metering-in isperformed as a function of operating parameters of the internalcombustion engine in a manner controlled by means of an engine controlunit (not illustrated) and a reducing agent metering device 3, in such away that the aqueous urea solution is injected into the exhaust-gaspartial flow via a nozzle 2. The nitrogen dioxide generated, asdescribed above, in the oxidation catalytic converter 4 is requiredfirstly for the reduction of the solid particles and secondly for thedownstream SCR reaction, as will be described in more detail below.

Downstream of the reducing agent injection point there is arranged aparticle filter 5 which accumulates the soot particles in the exhaustgas. The soot particles thus captured in the particle filter 5 arecontinuously converted, by means of the nitrogen dioxide generatedupstream with the aid of the oxidation catalytic converter 4, to formcarbon monoxide, carbon dioxide, nitrogen and nitrogen monoxide, suchthat cumbersome regeneration cycles for the particle filter 5 aredispensed with.

The actual selective catalytic reduction of the nitrogen oxides takesplace, as per the reaction described in EP 2014384 A2, by means of thesoot loading and an SCR catalytic converter 6 which is dulyschematically shown as a separate component in FIG. 1 for the sake ofsimplicity but which, as described further below, is formed as anSCR-active loading on the particle filter 5 in the reduction reaction inquestion, it is sought to convert as great as possible a fraction of thenitrogen oxides (NO_(x)) present in the exhaust gas into nitrogen andwater vapour while simultaneously achieving high selectivity of thereduction, without excess ammonia (NH₃) remaining present in theexhaust-gas flow. The method described in EP 2014384 A2 is expresslyincorporated into this description.

The intensely exothermic soot oxidation can result in a considerabletemperature rise, such that temperatures above 650° C., which lead todamage of SCR catalysts containing V₂O₅, may by all means occurdownstream of the particle separator. For this reason, the use of SCRcatalysts without V₂O₅, which are usually based on iron, cobalt orcopper zeolite material, may be expedient.

In the light of the constantly fluctuating operating conditions of aninternal combustion engine which is operated in a motor vehicle, it isobvious that the desired highest possible conversion rate for nitrogenoxides can be reliably attained only if a slight excess of ammonia isaccepted. In order, in situations of inadequate conversion, to preventpoisonous ammonia from being discharged with the partially purifiedexhaust gas into the ambient air, there is positioned downstream of theSCR catalytic converter an NH₃ oxidation catalytic converter 7 by meansof which the excess NH₃ is converted into nitrogen and water vapour. TheNH₃ oxidation catalytic converter 7, likewise shown schematically inFIG. 1 as a separate component, may likewise be part of the combinationof particle filter 5 and SCR catalytic converter, as presented furtherbelow. The oxidation reaction for the elimination of the excess ammoniashould take place as selectively as possible, such that at leastfractions of iridium or iridium oxide should be used as active materialfor the NH₃ oxidation catalytic converter 7.

As is also shown in FIG. 1 by the dashed circumferential line, ahydrolysis catalytic converter 8 may be used in order to improve thesplitting of the ammonia from the reducing agent if the temperaturelevel is not high enough at the supply point for the reducing agent. Thehydrolysis catalytic converter 8, arranged downstream of the supplypoint of the reducing agent and upstream of the particle filter 5 in theexhaust-gas flow, is likewise shown schematically as a separatecomponent in the drawing, but may also be a loading of correspondinglyactive catalyst material on the inflow side of the particle filter 5, aspresented further below.

The design of a particle filter with SCR activity is shown, likewiseschematically and in a sectional illustration, in FIG. 2, wherein theselected section plane runs parallel to the flow direction of theexhaust gas. The flow direction is indicated here by arrows.

The particle filter 9 has, both on its inflow side 11 and &so on itsoutflow side 12, blind-hole-like or pocket-like chambers 11 a, 12 a,wherein those which are situated on the inflow side 11 are open towardsthe inflow side 11 and those which are situated on the outflow side 12are open towards the outflow side 12. As filter material for theparticle filter 9, use may be made of cordierite or silicon carbide orelse sintered metal or ceramic fibres and silicate fibres or metalmeshes. The particle filter is coated or impregnated with SCR-activematerial such that it serves not only for the separation of the sootparticles but also as a catalytic converter for the reduction ofnitrogen oxides.

As can be seen in the illustration, the exhaust-gas flow which is ladenwith soot particles 10 and with ammonia (NH₃) (not illustrated) impingeson the inflow side 11 of the particle filter 9 and the soot particles 10are accumulated as a so-called filter cake 13 in particular in theblind-hole-like or pocket-shaped chambers 11 a, before the exhaust gaswhich has thus been purified of soot particles 10 flows through thefilter material to the blind-hole-like or pocket-like chambers 12 a ofthe outflow side 12. Before reaching the chambers 12 a, the exhaust gaswhich is laden with NH₃ flows through the SCR catalyst layer (notillustrated here), which is applied in and on the filter wall, until theexhaust gas flows out of the blind-hole-like or pocket-shaped chambers12 a. On the path through the layer laden with SCR-active catalystmaterial, and in the chambers 11 a and 12 a, the selective catalyticreduction as per reactions (1) and (2) takes place, wherein the nitrogenoxides are converted into nitrogen and water vapour. The exhaust gaswhich is thus purified of soot particles and nitrogen oxides exits thefilter arrangement on the outflow side 12.

For the regeneration of the particle filter 9, the filter cake 13 mustbe continuously removed. The continuous removal of the filter cake 13 isrealized, as already described with regard to FIG. 1, by virtue of anoxidation catalytic converter (not illustrated in FIG. 2) beingpositioned upstream of the particle filter 9, which oxidation catalyticconverter converts at least some of the nitrogen monoxide present in theexhaust gas into nitrogen dioxide as per reaction (2). The oxidationcatalytic converter must also be situated upstream of the point at whichthe reducing agent is metered into the exhaust-gas flow, becauseotherwise the reducing agent would be oxidized and thus unusable for theSCR reaction. The continuous regeneration of the filter cake isrealized, as already described above, by means of the periodicalternation between superstoichiometric and substoichiometric feedratio, and thus by means of a periodically high availability of thepowerful oxidant NO₂.

Upon contact with the filter cake 13, the nitrogen dioxide which isenriched in the exhaust gas effects a conversion of the soot particles10 to form CO, CO₂, N₂ and NO as per reactions (4), (5) and (6), whereinthe reactions take place in a fluctuating manner within the context ofthe NO₂ availability but continuously such that the filter cake can bekept at a substantially constant thickness and the exhaust-gas backpressure caused by the filter cake can be kept at a substantiallyconstant level.

For reasons of thermal stability of the SCR-active catalyst layer, itmay be advantageous for the SCR-active loading of the particle filter toincrease in the flow direction of the exhaust gas. This may be achievedeither by virtue of the particle filter being but up in layers, whereinthe loading of the layers with SCR-active material increases in the flowdirection of the exhaust gas, or by virtue of the loading with theSCR-active material being realized by means of coating or impregnationof the outflow side of the particle filter such that the degree ofloading of the filter material decreases with progressive penetrationdepth of the coating or impregnation solution.

In order to obtain as high as possible a conversion of nitrogen oxidesto form nitrogen and water vapour during transient operation of theinternal combustion engine, which is the normal situation in the case ofinternal combustion engines operated in vehicles, it is necessary tooperate the exhaust-gas aftertreatment system with a slight excess ofammonia. Since the poisonous ammonia gas must not be discharged into theambient air, it is necessary for an ammonia oxidation catalyst 17 to beprovided, for example as a coating, on the end of the exhaust-gasaftertreatment arrangement, as shown at the bottom in FIG. 2.

Possible production methods for the main body of the particle filter 3shall be presented, likewise by way of example and in principle, inconjunction with FIG. 2. To produce a blind-hole-like structure, it ispossible either for a multiplicity of tubular structures to be combinedto form a dense pack, wherein adjacent tubes in each case are closed offalternately either on the inflow side or on the outflow side. Theclosure may be realized by means of the same material as that which isalso used for the particle filter 9; the material may however alsodiffer, that is to say be neutral. A further possibility for theproduction of the blind-hole-like chambers 11 a, 12 a consists inproviding ducts in a block-like filter body, of which adjacent ducts ineach case are alternately closed off at the respectively opposite ends.

One possibility for producing pocket-like structures consists inproducing packs of plates which are spaced apart from one another,wherein the adjacent free spaces generated by the spacing are in eachcase alternately closed off on the inflow side 11 and on the outflowside 12, such that a pocket-like structure is formed.

As already stated in conjunction with FIG. 1, in order to improve thesplitting of ammonia from the reducing agent—for example aqueous ureasolution (AdBlue)—it may be necessary to use a catalytic converter whichpromotes the splitting in order to prevent the formation of cyanuricacid as per reactions (7) and (9). The formation of cyanuric acid is aproblem because it can result in blockage of the particle filter andthus to failure not only of the exhaust-gas aftertreatment system butalso of the internal combustion engine. To improve the splitting ofammonia from the reducing agent, the catalytic converter is producedsuch that, as schematically shown in the lower part of FIG. 2, theinflow side of the particle filter 9 is laden with a catalyst material16 which promotes the splitting. Materials which may be used for thispurpose include TiO₂ or TiO₂/SiO₂ or TiO₂/SiO₂/Al₂O₃, zirconium oxideand zeolites.

For more detailed explanation of the effects attained in the particlefilter 9 with the proposed method implementation, reference is madebelow to the illustration as per FIG. 3, which shows an arrangementsimilar to that shown in FIG. 2; the same reference symbols have beenused for identical parts such that a repeated description of the partscan be dispensed with, reference being made to the corresponding partsof the description relating to FIG. 2.

During the course of the loading with soot, a filter cake forms in thechambers 11 a, which filter cake, illustrated on an exaggerated scale,increases with progressive depth of the chambers 11 a. For theillustration of the processes, reference is made to the detailillustration at the bottom of FIG. 3. For the sake of simplicity, it isassumed that the particle filter 9 is impregnated uniformly withcatalyst material. In the filter material there are formed pores 14which form free passages through the filter material. Owing to the knowneffects of convection and diffusion and if appropriate thermophoresis,soot particles 10 accumulate in the chambers 11 a and also in the wallregions of the pores 14. If, owing to the ammonia storage effect, sootand SCR-active material of the particle filter 9 are now saturated withammonia, sufficient NH₃ availability is attained in the region of thesoot layer, of the ducts 14 and also in the chambers 11 a such thatnitrogen dioxide contained in the exhaust-gas flow, which nitrogendioxide was formed on the upstream oxidation catalytic converter, reactswith the ammonia as per equation (3). As a result, a depletion of NO₂occurs in this region, such that there is insufficient NO₂ available forthe oxidation of the soot particles as per equations (4) to (6). As aresult, the soot layer grows and leads to an increased exhaust-gas backpressure, which would be associated with a corresponding power loss ifthe particle filter 9 is installed in conjunction with an internalcombustion engine.

To counteract this, it is now possible to temporarily reduce theavailability of reducing agent, resulting in a depletion of NH₃proceeding from the duct net or from the untreated-gas side towards theclean-gas side, and consequently a shift of the SCR reaction away fromthe duct net or in the direction of the clean-gas side. Before all ofthe stored NH₃ has been consumed by the SCR reaction, the NH₃ store mustbe refilled, otherwise an intense increase of NO_(x) emissions wouldoccur downstream of the particle filter. This however has the resultthat, after such decrease phases, a disproportionately large amount ofreducing agent must be used in order to refill the ammonia store suchthat the reduction of the nitrogen oxides can take place to a sufficientextent. Since aqueous urea solution is used as reducing agent in motorvehicle applications, that is to say in conjunction with internalcombustion engines, the disproportionately high increase of the amountof reducing agent causes cooling of the exhaust gas and thus of theSCR-laden particle filter 9. Under critical operating conditions, thismay lead to incomplete conversion of aqueous urea solution into ammoniaowing to excessively low temperatures. The formation of intermediateproducts such as for example isocyanic add may then occur as perequation (8), which isocyanic add is then converted by trimerization, asper equation (10), into cyanuric add. The cyanuric add accumulates assolid matter on the particle filter and blocks the latter. Since suchdeposits can be dissolved again only with difficulty, this would lead,over a relatively long period of time, to failure of the internalcombustion engine.

To counteract this problem, the nitrogen oxide emissions of the internalcombustion engine are alternatively, or in combination, increased; thisis a measure which opposes that which is actually intended, because thenitrogen oxides are indeed that exhaust-gas constituent which is to beavoided. This is achieved in the case of an internal combustion engineby virtue of certain operating parameters being changed by way ofinterventions into the engine management, which is currently realized bymeans of electronic engine control systems. In the case ofair-compressing, auto-ignition internal combustion engines, for example,operating parameters which have a direct influence on the NO_(x)emissions include inter alia the start of injection for the fuelinjection, the air to fuel ratio (lambda), the fuel injection pressure,the number of fuel injections per working stroke, the intake airtemperature and the amount of exhaust gas recirculated (EGR rate) ifexhaust-gas recirculation is provided.

For example, an adjustment of the start of injection in the earlydirection, a reduction of the EGR rate, an increase of the fuelinjection pressure, of the air to fuel ratio or of the intake afttemperature, for example by virtue of the charge-aft cooler beingbypassed, leads to an increase of the NO_(x) untreated emissions.

Here, an increase of the NO_(x) emissions usually also entails anincrease of the combustion temperature, which has the effect that thetemperature level in the exhaust gas and consequently at the SCR-ladenparticle filter 9 increases. The opposite effect, therefore, to thatwhich would be attained in the case of a cyclic reduction and subsequentdisproportionately high increase of the amount of reducing agent that isadded (see above). This benefits the functional reliability of theparticle filter 9 because the formation of cyanuric add can be reliablyprevented. Furthermore, the conversion rate both of soot particles andalso of nitrogen oxides is positively influenced because the exhaust-gastemperature and particle filter/catalytic converter temperature can bekept in an optimum range.

The effect of the measure described above can be presented on the basisof a test example. For this purpose, a diesel engine of type MAN D2676was operated once with and once without the process implementationaccording to the invention, under constant load in each case; the amountof reducing agent remained unchanged. The increase of the NO_(x)emissions was performed cyclically every 3 seconds for 1 second in eachcase; the dosed amount of AdBlue and the integrated NO_(x) untreatedemissions were kept constant. The operating duration was 28 hours. As acomparative variable, the pressure loss across the particle filter wasdetermined after every hour of operation. The result of the comparisonis presented in the table in FIG. 4 and illustrated in diagram form inthe graph of FIG. 5. As can be seen from the table and the graphicillustration, without periodic NO_(x) increase, the pressure lossgenerated by the SCR-laden particle filter, measured in kPa, increasesrelatively sharply in the first 3 hours of operation from the initialvalue of 30 kPa, the pressure loss across the unladen filter, until afilter cake has formed in the filter; thereafter, the increase of thepressure loss duly slows, but not to zero. In the case of the periodicNO_(x) increase and the associated NO₂ increase resulting from the NH₃depletion on the untreated-gas side, a different picture emerges. Here,a slowed increase of the pressure loss is evident, and the pressure lossincreases no further after approximately 6 operating hours.

During the course of the test with periodic NO_(x) increase, noincreased NO_(x) fractions were encountered in the exhaust gas, asdetermined by measurements downstream of the catalytic converterarrangement.

The comparative data shows that, with the process implementationaccording to the invention, an increase of the pressure loss across theparticle filter 9 can be avoided without any change in the amount ofreducing agent, and blockage of the particle filter 9 can be reliablyprevented. An active, oxygen-based regeneration of the particle filterwith the associated disadvantages can be dispensed with. Since anintense cyclic decrease of the amount of reducing agent with subsequentdisproportionately high increase likewise does not take place, the riskof blockage of the particle filter as a result of inadequate reducingagent breakdown is likewise eliminated.

FIG. 6 is a flow diagram showing a process according to the presentinvention for an exhaust-gas aftertreatment system that operates onexhaust gas of an internal combustion engine operated with an excess ofair. According to the process, a particle filter separates sootparticles out of the exhaust gas, step 601. The particle filter isregenerated using NO₂ formed on an NO oxidation catalytic convertermounted upstream of the particle filter, step 602. Nitrogen oxides arereduced by metering a reducing agent, which splits to form ammonia, intoan exhaust-gas flow upstream of the particle filter, step 603. An NH₃ toNO_(x) ratio (feed ratio α) is periodically varied in phases by changingat least one of the nitrogen oxide (NO_(x)) untreated emissions and themetered amount of reducing agent, in such a way that the feed ratio αperiodically alternates in phases between values of greater than one andvalues of less than one, step 604. In an example described above, theperiodic variation of the feed ratio α is achieved by increasing theNO_(x) for one second every three seconds for an entire operatingduration of the engine.

The central concept of the proposed process implementation is that ofvarying the NH₃ to NO_(x) ratio (feed ratio α) in phases by varying theurea dosing and/or the nitrogen oxide untreated emissions. By means ofthis measure, as indicated in the detail illustration in FIG. 3, thestored ammonia is exhausted in phases in the soot layer and between thecatalyst surface indicated by the line 15 a and an imaginary line 15 bin the particle filter/catalyst material, such that as a result, an NH₃depletion zone 16 is generated in the particle filter/catalyticconverter and, as a result of the locally decreased NO_(x) conversion, asurplus of NO₂ is produced in the ducts 11 a and in those regions of thepores 14 which are adjacent to the catalytic converter surface. Thesurplus of NO₂ leads to an intensified oxidation of the stored sootparticles.

The process implementation illustrated by way of example above mayself-evidently be varied. It is for example possible to optimize theeffect of the method by virtue of the supplied amount of reducing agentnot being adapted correspondingly, in particular not being adaptedproportionally, to the periodically fluctuating NO_(x) untreatedemissions. A decrease of the amount of reducing agent is alsoconceivable here; it must however be ensured here, for example by meansof temperature measurement upstream of and/or at the combined particlefilter-SCR catalytic converter, that a predefined temperature level isnot undershot as the amount of reducing agent is increased again. It isin particular expedient to decrease the supplied amount of reducingagent, or to suspend the supply of reducing agent entirely, in thephases with high NO_(x) untreated emissions in order thereby toaccelerate the NH₃ depletion on the soot-laden side of the particlefilter.

It may furthermore be advantageous for the period length and/or themagnitude of the increase and/or the magnitude of the reduction and/orthe duration of the increase and/or the duration of the reduction of theNO_(x) untreated emissions to be selected as a function of the operatingconditions of the exhaust-gas aftertreatment system. Here, the catalyticconverter temperature and/or the degree of ammonia loading of thecatalytic converter and/or the NO_(x) conversion and/or the NO_(x)untreated emissions and/or the amount of NO₂ upstream of the particlefilter and/or the NO_(x) emissions downstream of the exhaust-gasaftertreatment system and/or the NH₃ emissions downstream of theexhaust-gas aftertreatment system and/or the supplied amount of reducingagent and/or the stored amount of NH₃ and/or the storable amount of NH₃and/or the degree of loading of the particle filter with soot may betaken into consideration as operating conditions. Such operatingconditions may be determined by means of sensors and/or by means ofmodels in the form of mathematical functions, characteristic maps and/orneural networks. Such techniques are known to a person skilled in theart, and therefore a detailed description is not required.

If unconsumed NH₃ passes the SCR catalytic converter, it may be providedthat the NH₃ is broken down by a material loading with an oxidativeaction arranged on the clean-gas side of the particle filter.

Furthermore, the proposed process implementation may be assisted byvirtue of the loading of the particle filter with SCR-active materialbeing realized such that the ammonia storage capability of the particlefilter laden with SCR-active catalyst material increases towards theclean-gas side. By means of this measure, those regions of the particlefilter laden with SCR-active catalyst material which are close to thesoot deposits can be placed into the NH₃-depleted state by means of arelatively small increase of the nitrogen oxide untreated emissions.

It must finally be expressly pointed out that an SCR-active coating orimpregnation of the particle filter is not necessary for theimplementation of the method according to the invention, because thesoot layer alone already exhibits the effect of an SCR catalyticconverter, and the method according to the invention is also effectivein an arrangement of this type.

The invention claimed is:
 1. A method for an exhaust-gas aftertreatment system that operates on exhaust gas of an internal combustion engine operated with an excess of air, the method comprising: separating, by a particle filter, soot particles out of the exhaust gas; regenerating the particle filter with the aid of NO₂ formed on an NO oxidation catalytic converter mounted upstream of the particle filter; reducing nitrogen oxides by metering a reducing agent, which splits to form ammonia, into an exhaust-gas flow upstream of the particle filter, wherein the particle filter is laden with a catalytically active material for selective catalytic reduction of nitrogen oxides; periodically varying an NH₃ to NO_(x) ratio (feed ratio α) in phases by changing at least one of the nitrogen oxide (NO_(x)) untreated emissions and the metered amount of reducing agent, in such a way that the feed ratio α periodically alternates in phases between values of greater than one and values of less than one.
 2. The method according to claim 1, wherein the catalytically active material includes one of soot deposited on the particle filter and an SCR catalyst applied to the particle filter.
 3. The method according to claim 2, wherein the SCR catalyst contains vanadium oxide or zeolite.
 4. The method according to claim 1, wherein a supplied amount of the reducing agent is not adapted correspondingly to periodically fluctuating NO_(x) untreated emissions.
 5. The method according to claim 1, wherein, during phases with high NO_(x) untreated emissions, a supplied amount of reducing agent is reduced or suspended entirely.
 6. The method according to claim 1, wherein the step of varying comprises raising the NO_(x) untreated emissions by one of changing the start of injection of the fuel, the air to fuel ratio, the fuel injection pressure, the number and chronological sequence of the individual fuel injections during a working stroke, the recirculated amount of exhaust gas, and the intake air temperature.
 7. The method according to claim 1, wherein at least one of the period length, the magnitude of the increase, the magnitude of the reduction, a duration of the increase, and a duration of the reduction of the NO_(x) untreated emissions is selected as a function of the operating conditions of the exhaust-gas aftertreatment system.
 8. The method according to claim 7, the operating conditions include at least one of a catalytic converter temperature, a degree of ammonia loading of the catalytic converter, an NO_(x) conversion, the NO_(x) untreated emissions, the amount of NO₂ upstream of the particle filter, the NO_(x) emissions downstream of the exhaust-gas aftertreatment system, the NH₃ emissions downstream of the exhaust-gas aftertreatment system, the supplied amount of reducing agent, the stored amount of NH₃, the storable amount of NH₃, and the degree of loading of the particle filter with soot.
 9. The method according to claim 7, further comprising the step of determining operating conditions of the exhaust-gas aftertreatment system by sensors or by models in the form of mathematical functions, characteristic maps, or neural networks.
 10. The method according to claim 1, further comprising the step of breaking down unconsumed NH₃ that passes the catalytically active material by a material loading with an oxidative action arranged on a clean-gas side of the particle filter.
 11. The method according to claim 1, wherein at least one of the NO_(x) untreated emissions and the feed ratio α is varied by at least 20% during said step of varying.
 12. The method according to claim 11, wherein at least one of the NO_(x) untreated emissions and the feed ratio α is varied by at least 40% during said step of varying.
 13. The method according to claim 11, wherein at least one of the NO_(x) untreated emissions and the feed ratio α is varied by at least 60% during said step of varying.
 14. An arrangement for carrying out the method according to claim 1, comprising: a particle filter laden with a catalytically active material; an NO oxidation catalytic converter mounted upstream of the particle filter; a metering device metering a reducing agent upstream of the particle filter, wherein loading of the particle filter with the catalytically active material increases from an untreated-gas side towards a clean-gas side of the particle filter, and wherein the arrangement is configured to periodically vary an NH₃ to NO_(x) ratio (feed ratio α) in phases by changing at least one of the nitrogen oxide (NO_(x)) untreated emissions and the metered amount of reducing agent, in such a way that the feed ratio α periodically alternates in phases between values of greater than one and values of less than one.
 15. The arrangement according to claim 14, wherein the loading of the particle filter with the catalytically active material is realized such that an ammonia storage capability of the particle filter laden with the catalytically active catalyst material increases towards the clean-gas side.
 16. The method according to claim 1, wherein the step of varying is performed during operation of the internal combustion engine.
 17. The method according to claim 1, wherein the step of periodically varying is performed every three seconds. 